1. Field of the Invention
The present invention relates to a stress-relieving heatsink structure for an electronic package, for instance, that includes a semiconductor chip which is mounted on a wired carrier, such as a circuitized substrate. Moreover, the invention is also directed to a method of attaching a stress-relieving heatsink structure to an electronic package.
The utilization of heatsink structures which are adapted to remove and dissipate heat generated by electronic packages, such as those comprising printed or wired circuit boards mounting one or more semiconductor chips is well known in the technology. Generally, heatsink configurations include either flat cap structures which are arranged above the semiconductor chip and heat conductively attached thereto; or heatsinks which are constituted of relatively thick and stiff base structures having pins or fins projecting therefrom in a direction or directions facing away from the electronic package or semiconductor chip so as to afford a good heat spreading capability while concurrently enabling the low cost manufacture thereof through the intermediary of suitable extrusion forming methods.
Ordinarily, in order to provide good thermal absorption of heat from the electronic package, the heatsink structure is adhesively attached through the interposition of a thin layer of a thermally conductive adhesive. However, depending upon the type of electronic package, thy stiffness of the base portion of the heatsink structure may prevent achievement of very high reliability of the electronic package due to a constraining effect produced by the structure thereof. The constraining effect can contribute to delamination and higher internal stresses, which can lead to eventual thermal fatigue of the electronic package components, in accelerated life testing.
Flexible electronic packages, (such as may have organic components in the form of the substrates or carriers mounting the integrated or printed circuits and forming integrated circuit boards), are more vulnerable to the constraining effect of the heatsink arrangement and can benefit by the use of a heatsink which does not create high stresses in the package and package interconnections.
In essence, heatsinks which have a high bending stiffness can contribute to high stresses during operation of the electronic package, which causes a significant amount of strain and stress to be imparted to the thermal adhesive which is employed in order to attach the heatsink to the electronic-package, whereby the relative deformation encountered among the various components causes failure in the operation thereof.
Currently employed heatsinks fail to provide for structures which are designed to adequately reduce stresses and strains generated during the operation of the electronic package as a result of generated heat, and which would tend to prevent delamination or fatigue of the electronic package components.
In particular, even present heatsink structures which are joined to each other fail to provide low stress joining arrangements in forming the combination thereof which would reduce stresses and strains to acceptable levels. These known structures do not address themselves to deriving the advantages which are obtainable by combining several heatsinks together prior to the assembly thereof with the electronic package components, which would require only a single location joining operation to be utilized during assembly, and a well controlled placement of the heatsinks relative to each other.
2. Discussion of the Prior Art
Thus, Hamburgen, et al. U.S. Pat. No. 6,034,430 discloses an integrated thermal coupling for a heat generating device wherein heatsink structures. include fins which are adapted to be interleaved to provide for enhanced heat dissipation. Although this provides an extensive degree of thermal contact between two separable heatsinks, the structure thereof is lightly complex, and fails to disclose the use of the effects of an adhesive interconnection between adjoining heatsink components, whereas contrastingly, the present invention teaches the mitigation of stress caused through the use of a heat conductive adhesive between the electronic package components and a composite heatsink arrangement.
Chaney, et al. U.S. Pat. No. 5,985,697 discloses heatsink consisting of a plate-like base member having a plurality of fins projecting therefrom, and which is adapted to dissipate heat generated by the electronic package. The structure disclosed in this patent fails to provide a multi-segmented heatsink arrangement which is adapted to reduce thermal mechanical stresses of the heatsink when coupled to the electronic package component.
Tzu, et al. U.S. Pat. No. 5,973,407 discloses a heat spreader arrangement which is integrally connected to a semiconductor package, and wherein a plurality of bumps and heatsink plate portions are adapted to disperse and dissipate heat to the exterior environment. There is no disclosure of a segmented heatsink arrangement which incorporates low-stress interconnections in order to avoid or reduce to tenable levels and the stresses and strains encountered during the generation of heat by the electronic package.
Similarly, Lee U.S. Pat. No. 5,641,987 discloses a heat spreader for semiconductor packages which includes different pad sizes adapted to conjointly dissipate heat generated by the package. However, as in the previously discussed prior art publications, there is no low-stress interconnection between various discrete heatsink segments which are combined to form an overall heatsink arrangement which would prevent delamination of the components of the electronic package caused by heat generated by the package.
Other patents for example, such as Lester U.S. Pat. No. 5,043,796 disclose multiple component heatsink structures which are interconnected in various regions so as to provide stress relief fracture paths. However, the stress-relief interconnections are merely constituted by reducing the thickness of part of the heatsink element to provide some bending ability, and does not in actuality produce a separate components which are adapted to be combined with other separate components to form a complete structure.
Similarly, Leecraft, et al. U.S. Pat. No. 4,296,455 also discloses a plurality of discrete heatsink components which do not, in actuality enable the reduction of stresses tending to delaminate the heatsink structure from the remaining portions of the electronic package to which they are fastened through the intermediary of an adhesive material, but which does not provide a unitarily connected entity.
Accordingly, pursuant to the present invention, there is provided a novel heatsink structure which incorporates a base structure incorporating stress-relieving slits forming discrete heatsink segments, whereby there is effectively reduced the constraining effect of a single large-sized heatsink with no appreciable loss in the thermal performance of the electronic package. Through the incorporation of low-stress adhesives, or possibly in some instances, snap fits, milled interlocking clearance slots, hinges or other low stress joining structures as the stress relieving slits provided intermediate various separate components of the heat sink structure or arrangement, a plurality of essentially smaller-sized separate heat sinks can be combined into one larger-sized low-stress heatsink structure which is pre-assembled and attached to the electronic package to enable the maximum heat-dissipation therefrom. Pursuant to preferred embodiments of the invention, an advantageous, choice for a low-stress adhesive material which may be used as connective material in the stress relieving slits intermediate the various separate heatsink substructures, there may be provided silicone or epoxy resin adhesives which are potentially constituted from the same type of material as the adhesive which is utilized to join the heatsink structure to the electronic package, such as the printed circuit or wiring board and the electronic component, such as the semiconductor chip or chips.
Accordingly, it is an object of the present invention to provide a novel heatsink structure for an electronic package which includes stress-relieving slits formed in the heatsink structure to form a plurality of essentially combinable discrete or separate smaller heatsink substructure, and which upon being adhesively attached to each other will provide for a stress-relieving arrangement while still maintaining good thermal conductivity in the heat dissipation from the electronic package.
Another object of the present invention is to provide a heatsink structure constituted of a plurality of combined smaller-sized heatsink devices or substructures which are joined along slitted edges through the intermediary of a low-stress adhesive or suitable engaging means, which will enable a certain degree of flexibility to be imparted to the overall heatsink structures so as to prevent delamination caused by heat-induced strains and stresses encountered during operation of the electronic component on which the heatsink is mounted, by a resultant reduction of the strains due to an enhanced flexibility and bendability of the heatsink structure.
Still another object of the present invention is to provide a method of forming the novel heatsink structure which is constituted of a plurality of joined or adhesively interconnected smaller heatsink components or substructures in a stress-relieving manner while the thermal conductivity of the electronic package is essentially maintained.